【 Weak current College 】 static analysis of fire and explosion---Power By 【 China power house network 】

In the chemical, oil refining, rubber, pharmaceutical, printing, metal powders production in industries such as electrostatic damage caused by accident is very large, it had attracted people's attention. Static electricity mainly in three areas, namely, the electrostatic discharge fire and explosion, lightning strikes and impede the production. Including electrostatic discharge fire and explosion is the most serious crimes against static electricity. In order to master the electrostatic discharge fire and explosion mechanism, first examine the characteristics of static electricity, can help you understand static electricity causes of accidents.
A, static characteristics

The electrostatic characteristics is closely associated with the static electricity, that is, static voltage characteristics and static leakage characteristics.

(A) voltage characteristics

Production process in an electrostatic charge is very small, in the local scope, electrostatic charge generally only microcoulombs to-Coulomb-level. However, the capacitor can be charged, to a large extent changes, sometimes very small, but the voltage u and Q capacitor C and power relations among the following:
ｕ=Q／C

Remain in power, voltage and capacitance inverse relationship maintained. The greater the voltage of the capacitor, lower; lower capacitance, voltage and higher. If you have two static objects are flat contact, during which the capacitance is equivalent to a flat on a flat plate capacitor, its size is:
C=εS／d

Type: S-flat area, d is the distance between the plates.

Suppose that two objects are dense contacts generate static, during which distance d1 = 25 × 10-8cm, when two objects, the separation distance when d2 = 0.1cm, and capacitance ratio is:

C1／C2=d2／d1＝0.1／25*10-8

This means that two objects after the separation, the capacitor is reduced to the original four live births, the voltage is increased to 400 times the original. Therefore, contact the electrostatic separation of high pressure is very dangerous.

For example: oil flowing in the pipeline, the voltage is not very high, but when injected into the tank, especially into the larger volume of tank, because gradually diminishing, capacitance and voltage rise significantly. In the event of an electrostatic discharge, will cause a fire or explosion.

2. electrostatic discharge fire and explosion

From domestic and foreign large amounts for electrostatic fire and explosion of analysis that: electrostatic discharge fire and explosion, you must have a combustible, explosive substance or mixture combustion, this is a necessary condition for the fire; and second, you must have to breakdown voltage, the dielectric caused by electrostatic discharge, generate static spark; the third is the electrostatic discharge energy must be equal to or greater than the substance of the minimum ignition energy, become the material of the cited source of fire. These three are electrostatic discharge fire and explosion of the most basic conditions are as follows:

(A) combustible or explosive mixtures

Fuel is where with oxygen or other antioxidants play a violent reaction of the substance. Such as wood, paper, gasoline, acetylene, etc. Where can help and support for the combustion of substances known as combustion, such as air, oxygen, potassium permanganate, etc.

Explosive mixture is air and combustible gas or liquid vapor-phase hybrid, meet fire can be a mixture of the explosion. Explosive mixtures have explosive limit and the lower explosion limit, when an explosive mixture of concentration in the lower range of explosion, the encounter with the fire can cause a fire and explosion.

(Ii) electrostatic discharge energy

Electrostatic discharge energy is the spark discharge electrostatic field through the release of energy. Electrostatic discharge-calculation under the energy available.
W=1／2Cu2

Type: W energy for electrostatic discharge Sparks, C as object of electrostatic capacitance, u is an object's live potentials.

When one objects generate static, its discharge energy must be equal to or greater than the substance of the minimum ignition energy, will cause a fire or explosion. Substance of minimum ignition energy refers to the material to ignite the minimum ignition energy. Saturated hydrocarbons and their derivatives of minimum ignition energy mostly 0.2mJ orders of magnitude, but the minimum ignition acetylene energy only 0.019mJ, only carbon disulfide 0.009mJ, etc. Industrial dust of minimum ignition energy generally is between 10-100mJ; gas and steam explosive mixture of minimum ignition energy more in 0.009-0.29mJ.

According to the test, the minimum ignition of methane to energy, if one is wearing 0.28mJ rubber shoes of workers, in a place full of methane gas, its off of work clothing fibres, volunteers bring about potential 3kV, if human electrostatic capacitance of 100 × 10-12F, when a part of the body touches the ground objects and so on, then discharge the spark energy is:

W＝1／2Cu2=0.5*100*10-12*30002=0.45(mJ)

When the spark discharge energy 0.45mJ will cause methane gas combustion and explosion.

From electrostatic discharge fire and explosion of the three basic criteria, are mutually dependent, are indispensable. If any combustible or explosive mixtures and produce the energy equivalent static discharge, the substance of minimum ignition energy, combustion will not occur. In contrast, produces the energy equivalent static discharge, combustible or explosive mixtures in small numbers or concentrations is not, it is not burning. According to this reasoning, we can take some measures to prevent the occurrence of electrostatic fire. For example for fire and explosion hazard workshop, take the local exhaust ventilation or all methods to reduce flammable gas, vapour or dust in the workshop, the concentration in the air so that you can avoid fire accidents.